Carbon-based conductive ink

Abstract

The present invention provides a liquid composition comprising conductive carbon particles and / or carbon nanoparticles, a thickener, and a solvent. The carbon nanoparticles are preferably a mixture of graphite nanoplatelets and carbon nanotubes, and the thickener is preferably a cellulose derivative. The liquid composition can be used as an ink for printing highly conductive films that adhere to paper substrates.

Description

[Technical Field]

[0001] The present invention relates to conductive inks containing carbon nanomaterials, methods for making such inks, their uses, and substrates onto which the conductive inks are printed. [Background technology]

[0002] Two-dimensional (2D) materials are crystalline materials composed of several layers or monolayers (monolayers) of atoms or molecules. A wide range of 2D materials is known, including graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs). The chemical formula of TMDs is MX2, where M is a transition metal and X is a chalcogen atom (S, Se, or Te). Examples of such TMDs include molybdenum disulfide (MoS2), niobium diselenide (NbSe2), and tungsten disulfide (WS2).

[0003] 2D materials are known to have many interesting and potentially useful properties that differ from those of their bulk 3D counterparts. For example, graphene is highly conductive, leading to applications in conductive composites as well as electrode structures.

[0004] Interesting functional properties of many materials are often only observed when the materials are in a single- or few-layer (i.e., 2D) form. However, exfoliation of bulk three-dimensional (3D) materials to form their 2D counterparts requires overcoming strong interlayer dispersion forces.

[0005] Carbon nanotubes are nano-sized tubes composed of rolled sheets of graphene. The tubes typically range in diameter from 1 to 50 nanometers, but can also have lengths in the micrometer range. Carbon nanotubes can be either single-walled (i.e., formed from a single rolled sheet of graphene) or multi-walled (i.e., formed from multiple concentric rolled sheets of graphene). Carbon nanotubes have attracted significant interest due to their physical properties, namely, high tensile strength and high electrical conductivity.

[0006] Dispersions containing carbon nanomaterials (e.g., carbon nanotubes, carbon nanographite, graphene, and mixtures thereof) have been considered as inks that can be used to deposit conductive films. Such films have the advantage of being "metal-free" for certain commercial applications, yet still conductive. However, to date, the use of such inks has been limited to printed films with low electrical conductivity. For example, copper has a conductivity of 6×10 7 Although carbon nanomaterials have conductivities in the range of 100 S / m, reported films made from carbon nanomaterials typically have conductivities much lower than 100 S / m (see U.S. Patent No. 10,244,628). Furthermore, existing printed carbon-containing inks can only be printed on a limited range of substrates, such as aluminum and plastics (especially polyethylene terephthalate (PET)). These materials cannot be recycled.

[0007] The formulation of printable inks based on dispersions of carbon nanomaterials in water has suffered from aggregation issues due to the non-polar nature of these materials, which reduces their industrial applications due to the need for settling of the nanocarbon materials and excessive organic solvents.

[0008] Khan et al., "The preparation of hybrid films of carbon nanotubes and nano graphite / graphene with excellent mechanical and electrical properties", Carbon 48 (2010), pp. 2825-2830, report that hybrid films containing both carbon nanotubes and nano graphite have higher electrical conductivity than films containing only one component. However, Khan et al. only describe the dispersion of nano graphite and carbon nanotubes in N-methylpyrrolidone solvent. The solvent is removed by vacuum filtration to form a film of carbon nanomaterials. The electrical conductivity of the film is up to 2 × 10 4It is only S / m and this fluid is not suitable for printing.

[0009] Pan et al., "Sustainable production of highly conductive multilayer graphene ink for wireless connectivity and IoT applications," Nature Comm. (2018), 9:5197, describes an ink containing graphene, dihydrolevoglucosenone, and NMP. The conductivity of films printed from these inks is only 7.13 × 10 4 It was S / m.

[0010] Ferrari et al. (WO2017 / 060497A1) reported that the 4 We describe the fabrication of liquid-phase exfoliated GNP / carboxymethylcellulose films that exhibit a conductivity of 100 S / m. These films were printed onto a PET substrate and used to fabricate UHF RFID tags with a read range of 1.4 m at 2 W incident radiation.

[0011] Environmentally friendly recycling of mixtures is a perennial challenge, especially in the electronics industry. Implementing electronic systems with less negative environmental impact stimulates new innovations in the material combinations used in the manufacture of these devices. Mass-produced UHF RFID tags are composed of mixed materials (plastic, metal, silicon, and paper). Moving towards materials with increased environmental reliability and acceptable performance is of interest to many stakeholders. In some cases, metals are not preferred due to strict requirements for product screening to protect consumer interests.

[0012] High solids inks are an essential requirement to reduce the environmental impact of printing due to the drying process. Stabilizing nanocarbon dispersions using co-binders increases the potential thickness of screen-printed films. This helps reduce resistive losses, which are essential for various printed electronics applications. For efficient carbon-based RF antenna applications, the printed film thickness must be thinner than this, typically limited to less than 100 μm due to process and ink solids considerations (Jordan, Edward Conrad (1968), Electromagnetic Waves and Radiating Systems, Prentice Hall, ISBN 978-0-13-249995-8). Summary of the Invention [Problem to be solved by the invention]

[0013] There remains a need for alternative structures based on alternative carbon-based conductive inks that preferably have improved conductivity and / or can be printed on recyclable substrates. [Means for solving the problem]

[0014] The inventors of the present application have demonstrated that printable inks comprising graphite nanoplatelets or graphite particles and single-walled carbon nanotubes have very high electrical conductivity (up to 5×10 5 kS / m, see Examples 2 and 6). Such inks can be used in a wide variety of applications, including the production of "metal-free" antennas and printed heaters for RFID tags.

[0015] Thus, in a first aspect, the present invention provides a method for producing a medicament for the treatment of a medicament comprising: (i) carbon nanomaterials, (ii) a thickening agent, and (iii) a solvent. The thickener can adequately bind the carbon nanomaterial and adhere to the substrate, such as a cellulosic substrate or other suitable hydrophilic substrate. The thickener can be or include a cellulose derivative. The inventors have also found that it is possible to prepare a carbon nanomaterial-containing ink that can be printed and adhered to recyclable substrates, particularly paper.

[0016] The composition may comprise carbon nanotubes as the carbon nanomaterial or as one of the carbon nanomaterials. The composition may also comprise conductive carbon particles. Preferably, the composition comprises a mixture of carbon nanotubes and further conductive carbon particles.

[0017] Thus, in a second aspect, the present invention provides a method for producing a medicament for the treatment of a medicament comprising: (i) conductive carbon particles; (ii) carbon nanotubes, (iii) a thickening agent, and (iv) a solvent.

[0018] The thickener properly separates and encapsulates the carbon nanotubes, providing a means of dispersion for a maximum number of individual conductive pathways between the nanotubes and the conductive carbon particles.

[0019] In some embodiments, the conductive carbon particles are graphite particles, for example, micron-sized graphite particles.

[0020] In another embodiment, the conductive carbon particles are graphite nanoplatelet particles. The inventors have advantageously discovered that when the carbon nanomaterial is a mixture of graphite nanoplatelets and single-walled carbon nanotubes and the thickener is a cellulose derivative, films printed from these liquid ink compositions have high electrical conductivity.

[0021] Thus, in a third aspect, the present invention provides a method for producing a medicament for the treatment of a malaria parathyroidectomy, comprising: (i) graphite nanoplatelets, (ii) carbon nanotubes, (iii) cellulose derivatives, and (iv) a solvent.

[0022] The liquid composition dries (after being printed) to form a conductive film that can adhere to a cellulose-containing substrate. When the solvent is aqueous, the composition may more accurately be referred to as a hydrogel ink due to the nature of the interaction between the cellulose-derived thickener and the solvent. Reference herein to a liquid composition of the present invention encompasses hydrogel ink unless the context requires otherwise.

[0023] The liquid compositions described above may also be provided in dry powder or aerogel compositions in the absence of solvent.

[0024] In a further aspect of the invention, (i) conductive carbon particles (e.g., carbon nanomaterials), and (ii) A substrate (e.g., a cellulosic substrate) is provided that is printed with a conductive ink that includes a binder that binds to cellulose, suitably a cellulose derivative.

[0025] The present invention also provides a method for preparing a substrate (e.g., a cellulosic substrate) comprising: i) conductive carbon particles (e.g., carbon nanomaterials); ii) cellulose derivatives, and iii) a solvent.

[0026] It has also been shown that the liquid compositions described herein can be printed onto a stretchable substrate. Also provided are compositions that can be printed onto a stretchable substrate, as described in more detail herein.

[0027] As mentioned above, the ink may contain carbon nanotubes, or may contain graphite particles as the conductive carbon particles. Similar improvements in conductivity have been observed with graphite particles as with graphite nanoplatelets (see Example 6 below).

[0028] Compared to the films described in WO2017 / 060497, the present invention achieves up to 5×10 5 S / m -1 The results show a significant improvement in overall film conductivity, with higher overall film conductivity. Films printed according to the present invention have also been shown to be capable of being printed on common cellulosic substrates (e.g., paper) and stretchable substrates with good film-forming and stability properties.

[0029] The concentration of ink solids and the use of screen printing also facilitate the thick film formation necessary to achieve good electrical conductivity (0.1 ohm / sq / mil), giving the film suitable antenna properties and the necessary electromagnetic "skin depth" characteristics required for radiating antennas within the UHF band.

[0030] Printing conductive structures onto substrates enables a variety of applications through the integration of surface-mounted electronic components. Examples of potential commercial applications include RFID tags, microheaters, and sensors. [Brief explanation of the drawings]

[0031] [Figure 1] FIG. 1 shows a scanning electron microscope (SEM) image of the printed ink described in Example 2 below. [Figure 2] FIG. 2 shows a scanning electron microscope (SEM) image of the printed ink described in Example 2 below. [Figure 3] FIG. 3 is a photograph of the printed antenna described in Example 3 below. [Figure 4] Figure 4 is a photograph showing the flexibility of the printed antenna shown in Figure 3. [Figure 5] FIG. 5 is a photograph showing the print resolution of the ink described in Example 2 below. [Figure 6] FIG. 6 shows the strain response of the resistance of the printed film described in Example 4. [Figure 7] FIG. 7 shows the shape of the antenna pattern printed in Example 4. [Figure 8] FIG. 8 shows the resonant frequencies of films printed in Example 4 with different GVL contents. [Figure 9] FIG. 9 shows the resonant frequency of the film described in Example 4 as a function of strain. [Figure 10] FIG. 10 shows the strain response of the resistance of the printed films described in Example 4 (see FIG. 6) along with the estimated resistance of these printed films. [Figure 11] FIG. 11 shows the printed film described in Example 8 for use as a printed heater. [Figure 12] Figures 12A and 12B show infrared thermal images of the printed film of Figure 11 when no potential difference is applied to the film (Figure 12A) and when a potential difference of 10 V is applied to the film (Figure 12B). [Figure 13] FIG. 13 shows the conductivity versus mass fraction of carbon nanotubes from a film printed from an ink containing graphite particles and carbon nanotubes, as described in Example 6 below. [Figure 14] FIG. 14 is a schematic diagram showing the width, length, and thickness parameters of graphite nanoplatelets, respectively. [Figure 15] FIG. 15 is a rheology trace showing the viscosity of the ink described in Example 2 below. DETAILED DESCRIPTION OF THE INVENTION

[0032] The term conductive carbon particles refers to particles that contain carbon, are electrically conductive, and have a conductivity of 750 S / m or more, such as 1000 S / m or more.

[0033] The conductive carbon particles typically comprise greater than 80% by weight carbon, preferably greater than 90% by weight carbon, for example greater than 95% by weight carbon. In some compositions described herein, the conductive carbon particles consist of carbon (i.e., comprise carbon and little or no other elements).

[0034] As mentioned above, conductive carbon particles are conductive. Therefore, sp 2 The proportion of carbon atoms in the hybrid conductive carbon particles is usually 50% or more, for example 75% or more, and preferably 90% or more.

[0035] Examples of conductive carbon particles include graphite and graphene (e.g., graphite nanoplatelets). Accordingly, the average particle size can be on the micron scale or nanoscale, respectively.

[0036] When conductive carbon particles (e.g., graphite particles) are micron-scale, they typically have dimensions of 1 μm or more, e.g., 2 μm or more or 3 μm or more, in all three dimensions (length, width, and thickness). However, micron-scale conductive carbon particles typically have a longest dimension of 50 μm or less, usually 30 μm or less, e.g., 25 μm or less or 20 μm or less.

[0037] As used herein, the term "carbon nanomaterial" refers to a nanomaterial containing or consisting of carbon (i.e., a material having one critical dimension with an average size between 1 nm and 100 nm). Typically, a carbon nanomaterial contains at least 90% by weight, preferably at least 95% by weight, e.g., 99% by weight or more, of carbon. This term includes materials such as graphene, graphite nanoplatelets, single-walled carbon nanotubes, multi-walled carbon nanotubes, crystalline diamond, and diamond-like carbon (see ISO standard ISO / TS80004-3:2010). Typically, a carbon nanomaterial is a conductive carbon nanomaterial. Preferably, the carbon nanomaterial comprises a mixture of (i) graphene nanoplatelets and (ii) single-walled carbon nanotubes, multi-walled carbon nanotubes, or both. It is particularly preferred that the carbon nanomaterial comprises a mixture of (i) graphite nanoplatelets and (ii) single-walled carbon nanotubes. The dimensions of the nanomaterial can be determined by transmission electron microscopy.

[0038] We have found that a cooperative effect on electrical conductivity exists in compositions containing both graphite particles or graphite nanoplatelets and single-walled carbon nanotubes. Without wishing to be bound by theory, we believe that carbon nanotubes provide conductive bridges between individual graphite particles or graphite nanoplatelets, thus reducing the "patch resistance" of individual nanoplatelets / particles. Patch resistance is caused by finite tunneling of electrons between adjacent sheets and is much higher than the movement within the internal structure of a sheet (graphite) or rod (carbon nanotube). Furthermore, without wishing to be bound by theory, we believe that the junction resistance between a graphite nanoplatelet or particle and a carbon nanotube is lower than the junction resistance between two nanoplatelets / particles or two nanotubes. Therefore, intimate mixing of nanoplatelets / graphite particles with nanotubes improves the electrical conductivity of films formed from the liquid compositions described herein that contain both graphite nanoplatelets / particles and carbon nanotubes (specifically, single-walled carbon nanotubes).

[0039] To maximize this effect, the carbon nanotubes are preferably individualized. Typically, greater than 75 wt. %, e.g., greater than 80 wt. %, and preferably greater than 85 wt. % of the nanotubes in the composition are individualized. Individualized nanotubes can be seen in Figures 1 and 2. The degree of nanotube individualization can be determined by UV-visible spectroscopy, as individualized single-walled carbon nanotubes exhibit van Hove singularities (peaks) at specific wavelengths (Alafogianni et al., Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 495, (2006), pp. 118-124). These UV-visible absorption peaks are not visible in bundled carbon nanotubes, so the prominence of these peaks is a measure of exfoliation / individualization.

[0040] The loading of particles of various insoluble geometric shapes and sizes can enhance a wide range of physical properties, depending on the nature of those particles. This effect extends to the nanoscale. By carefully combining different particle sizes and shapes, the overall physicochemical properties of the formulated system can be tailored to achieve desired properties. For commercial applications, the cost of the most active components necessitates significant loadings (>50%) of systems with lower-cost fillers that do not unacceptably affect performance or are added to impart additional properties such as thermal conductivity, mechanical strength, and / or chemical reactivity. In this invention, cost-effective formulations can be achieved by concentrating the most conductive hydrogel components within the filled voids within a matrix of larger conductive carbon particles (one dimension of which may be nanoscale). Blending thixotropic single-walled carbon nanotube hydrogels with conductive carbon particles ensures that high conductivity is maintained throughout the printing and drying process, resulting in excellent film conductivity.

[0041] As used herein, the term "graphite nanoplatelets" (also referred to herein as "graphene nanoplatelets") refers to nanoparticles of graphite composed of small stacks of graphene. The term "few-layer" nanoplatelets refers to nanoplatelets having an average of 20 layers or less, typically 15 layers or less, and preferably 10 layers or less. The number of layers can be determined by UV-Vis spectroscopy (see C. Backes et al., "Spectroscopic metrics allow in-situ measurement of mean size and thickness of liquid-exfoliated graphene nanosheets," Nanoscale, 2016, doi: 10.1039 / C5NR08047A).

[0042] Nanoplatelets typically have an average thickness of less than 30 nm, e.g., less than 20 nm. As used herein, the term "thickness" refers to the dimension of the nanoplatelet along the stacking axis of the layers within the nanoplatelet. The terms "length" and "width" refer to the long and short dimensions of the nanoplatelet, respectively, along orthogonal axes in the plane of the sheet of laminated material (see Figure 14). Nanoplatelets typically have an average length and / or width of 30 nm or more, preferably 50 nm or more or 100 nm or more. Nanoplatelets typically have an average length and / or width of 3.0 μm or less, e.g., 2.0 μm or less, usually 15 μm or less, preferably 1 μm or less, e.g., 800 nm or less. Nanoplatelet dimensions can be measured using scanning or transmission electron microscopy. In contrast to the micron-sized particles described above, which have three dimensions that are micron-sized (i.e., length, width, and thickness all equal to or greater than 1 μm), nanoplatelets typically have two dimensions that are micron-sized (i.e., length and width that are micron-sized, and a thickness that is significantly less than 1 μm, e.g., less than 100 nm). As previously mentioned, these dimensions can be measured by transmission electron microscopy.

[0043] When the conductive carbon particles comprise graphite nanoplatelets, the graphite nanoplatelets are typically present in the liquid composition in an amount of from 0.5% (w / w), preferably from 0.75%, such as from 1% to 5% (w / w), preferably from 3% (w / w), for example, to 2% (w / w). When the liquid composition is dried to form a dry film / powder, the graphite nanoplatelets are typically present in an amount of from 25% (w / w), preferably from 30% (w / w), such as from 35% (w / w), and / or to 50% (w / w), preferably to 45% (w / w), for example, to 40% (w / w).

[0044] When the conductive carbon particles comprise micron-sized graphite particles, the graphite particles are typically present in the liquid composition in an amount of 0.5%, such as 1% to 5% (w / w), preferably up to 3% (w / w), such as up to 2% (w / w). When the liquid composition is dried to form a dry film / powder, the graphite particles are typically present in an amount of from 30% (w / w), preferably from 40% (w / w) to 70% (w / w), preferably up to 55% (w / w), such as up to 60% (w / w).

[0045] The carbon nanotubes may be single-walled or multi-walled carbon nanotubes, but preferably comprise or consist of single-walled carbon nanotubes. Carbon nanotubes typically have an average outer diameter of 1 nm to 5 nm, preferably 1 nm to 2 nm (as measured by transmission electron microscopy), and may have a length greater than 3 μm, usually greater than 5 μm, e.g., greater than 10 μm or greater than 15 μm. While the micron-sized particles described above are micron-sized in three dimensions and nanoplatelets are micron-sized in two dimensions, carbon nanotubes are micron-sized in only one dimension (i.e., along their length).

[0046] The carbon nanotubes may be present in the compositions described herein in a weight ratio to the amount of graphite nanoplatelets or graphite particles of greater than 0.05:1 (carbon nanotubes:graphite nanoplatelets / particles), such as greater than 0.10:1 or 0.15:1, preferably greater than 0.2:1, up to 1:1, suitably up to 0.75:1 or up to 0.5:1, such as up to 0.4:1 or up to 0.35:1.

[0047] For example, when the conductive carbon particles are graphite nanoplatelets, the carbon nanotubes are typically present in the formulation in a weight ratio to the amount of graphite nanoplatelets of 0.15:1 to 0.7:1 (carbon nanotubes:graphite nanoplatelets), preferably 0.2:1 to 0.6:1.

[0048] Alternatively, when the conductive carbon particles are micron-sized graphite particles, the carbon nanotubes are typically present in the formulation in a weight ratio to the amount of graphite particles of 0.02:1 to 0.2:1 (carbon nanotubes:graphite particles), preferably 0.05:1 to 0.15:1.

[0049] Alternatively, the amount of carbon nanotubes in the composition can be defined relative to the weight of the entire composition. For example, the carbon nanotubes can be present in the liquid composition in an amount of from 0.1% (w / w), preferably from 0.25%, for example, from 5% to 1.5% (w / w), preferably from 1.25% (w / w), for example, up to 1% (w / w). When the liquid composition is dried to form a dry film / powder, the carbon nanotubes are typically present in an amount of from 5% (w / w), preferably from 10% (w / w), for example, from 15% (w / w) to 30% (w / w), preferably up to 25% (w / w), for example, up to 20% (w / w).

[0050] The solvent can be aqueous or non-aqueous. However, the solvent is preferably water (necessary for hydrogel formation) or contains water. Alternatively, the solvent can be a dipolar aprotic solvent. Examples of such dipolar aprotic solvents include cyclopentanone, cyclohexanone, N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), sulfolane, dihydrolevoglucosenone (Cyrene), and lactones such as gamma-valerolactone. We have found that a solvent system containing a combination of water and gamma-valerolactone provides an ink suitable for printing on stretchable substrates (see Example 4 below).

[0051] The composition may also include a thickener (which may also act as a gelling agent) to increase the viscosity of the composition, which ensures that the composition is suitable for printing and also reduces the tendency of the carbon nanomaterials to agglomerate.

[0052] The thickener is preferably a hydrogel-forming thickener. As mentioned above, highly conductive inks are obtained by forming a hydrogel matrix containing carbon nanotubes and conductive carbon particles. Hydrogel-forming thickeners are generally hydrophilic polymer chains that form colloidal gels in water through extensive hydrogen-bonding networks.

[0053] The thickener also preferably binds to cellulose, for example, when the ink / liquid composition of the present invention is printed onto a cellulose-containing substrate (such as paper) and dried.

[0054] Examples of suitable thickening agents include: Cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, hydroxyethyl cellulose, and carboxyethyl cellulose, and their salts (sodium salts, etc.) Polymers such as polyethylene oxide (PEO) and polypropylene oxide (PPO) Polyanaline (PANI), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and poly N-isopropylacrylamide (PNIPAAm) Cyclodextrin Natural gelling agents such as xanthan gum, gelatin, glycerol, alginate, and chitosan Inorganic silica and clays such as bentonite, montmorillonite, laponite, nanosilica, and titania Fibrous or rod-like materials, e.g., those with an aspect ratio greater than 100 (e.g., carbon nanotubes)

[0055] In a preferred embodiment, the thickening agent is a cellulose derivative, such as carboxymethylcellulose. The term cellulose derivative, as used herein, refers to a chemical derivative of cellulose formed by functionalizing some or all of the hydroxyl groups present in cellulose (e.g., by etherification or esterification). Derivatives can be formed by incorporating one or more or all of the following groups: carboxy, hydroxy, methyl, ethyl, and / or propyl groups. Examples of cellulose derivatives include hydroxypropylmethylcellulose, hydroxypropylcellulose, methylethylcellulose, methylcellulose, and carboxymethylcellulose, or combinations thereof, as well as cellulose itself. Liquid ink compositions containing this type of binder have been found to adhere well to paper substrates. CMC is available in several forms (e.g., depending on the degree of substitution and functionality) and can be crosslinked with several chemicals via covalent bonds or hydrogen-bonding networks with other agents to impart new properties that can be tailored to specific requirements (Gels 2018, 4, 54; doi: 10.3390 / gels4020054).

[0056] Cellulose derivatives readily form hydrogels, which are utilized in many industrial applications. These materials can also function as surfactants to stabilize nanocarbon materials in aqueous media. Hydrogels exhibit ideal thixotropic behavior due to extended hydrogen bonding or supramolecular network formation. These networks help provide long-range order for improved rheological behavior.

[0057] The total concentration of thickener may range from 0.5% to 2% by weight of the total composition (including solvent), for example from 1% to 1.75% by weight of the total composition.

[0058] The thickener is believed to increase the viscosity of the composition and also allow the carbon nanotubes, if present, to form a pre-ordered supramolecular network, thereby increasing the electrical conductivity of films printed from the composition.

[0059] The viscosity of the composition is important to ensure it can be printed to form a film. Additionally, the composition must be viscous enough to prevent aggregation of the carbon nanomaterials in the composition. Of course, the exact viscosity depends on the intended use of the composition (and resulting film). The thickener also ensures that the ink has a suitable viscosity for printing, such as screen printing. Inks suitable for screen printing are typically thixotropic, and therefore their viscosity depends on the shear rate. As shown in Figure 15, the ink can have a viscosity of 100-1000 Pa·s at a shear rate of 0.1 / s and / or a viscosity of 1-10 at a shear rate of 100 / s.

[0060] The composition may also contain one or more surfactants.Surfactants are typically nonionic surfactants.Examples of suitable nonionic surfactants include polyethylene oxide (PEO) surfactants (e.g., Triton X-100), polypropylene oxide (PPO) surfactants, cyclodextrins, and polyvinylpyrrolidone (PVP) surfactants.However, ionic surfactants such as sulfate surfactants (e.g., sodium dodecyl sulfate) can also be used.

[0061] The total concentration of surfactant may range from 0.01% to 1% by weight of the total composition (including solvent), or from 0.01% to 0.1% by weight, for example from 0.02% to 0.05% by weight of the total composition.

[0062] The composition may also include one or more solvents and / or adhesives to improve adhesion of the dry film (formed by printing the ink) to the substrate. The nature and combination of adhesives will, of course, vary depending on the substrate.

[0063] The composition may also contain one or more crosslinkers to improve the rheological parameters of the ink and / or the properties of the resulting film. This can include a wide range of functional organic acids or bases, such as ascorbic acid. Additional examples of crosslinkers include dicarboxylic and tricarboxylic acids, such as glutaric acid and trimesic acid. This crosslinking serves to stabilize the film from rapid redissolution and the effects of ambient humidity on conductivity.

[0064] In addition, the composition can further include a curing agent, which is a material that cures upon exposure to heat or radiation to harden the liquid ink composition into a solid film. These include photocurable monomers or infrared activators, such as epoxides (which may undergo ring-opening reactions), aldehydes, or acids such as citric acid (which may undergo esterification reactions).

[0065] In an exemplary embodiment, the invention provides a composition comprising: (a) Graphite nanoplatelets or graphite particles (b) Carbon nanotubes (c) Cellulose derivatives (d) Surfactants (e) water

[0066] In one embodiment, the present invention provides a composition comprising: (a) Graphite nanoplatelets or graphite particles (b) Carbon nanotubes (c) Carboxymethylcellulose (d) Triton X-100 (e) water

[0067] In a further embodiment, the present invention provides a composition comprising: (a) Graphite nanoplatelets in the weight range of 0.5% to 3% (w / w) (b) Carbon nanotubes in the weight range of 0.1% to 1.5% (w / w) (c) carboxymethylcellulose in the weight range of 0.5% to 2% (w / w) (d) Triton X-100 in the weight range of 0.01% to 0.1% (e) water

[0068] In still a further embodiment, the present invention provides a composition comprising: (a) Graphite nanoplatelets or graphite particles (b) Carbon nanotubes (c) Carboxymethylcellulose (d) γ-valerolactone (e) Triton X-100 (f) water

[0069] In a further embodiment, the present invention provides a composition comprising: (a) Graphite nanoplatelets in the weight range of 0.5% to 3% (w / w) (b) Carbon nanotubes in the weight range of 0.1% to 1.5% (w / w) (c) carboxymethylcellulose in the weight range of 0.5% to 2% (w / w) (d) γ-valerolactone in the weight range of 3% to 7% (w / w) (e) Triton X-100 in the weight range of 0.01% to 0.1% (f) water

[0070] A preferred component of the liquid composition is a cellulose derivative, as described elsewhere. Ethyl cellulose, methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and hydroxyethyl cellulose are suitable. Carboxymethyl cellulose (CMC) and its derivatives are particularly suitable. Salts of carboxymethyl cellulose, such as the sodium salt, can also be used.

[0071] In our testing, CMC was found to provide the composition with a strong binding affinity for cellulosic materials such as paper and card, and was expected to bind to cotton as well, making it ideal for these substrates. During use, CMC formed a stable hydrogel with water, resulting in a printable, highly conductive ink that adhered to paper.

[0072] In a further aspect, the present invention provides a method of making an ink, comprising: (i) Obtaining exfoliated graphite nanoplatelets (ii) Obtaining exfoliated single-walled carbon nanotubes (iii) dispersing the exfoliated graphite nanoplatelets, exfoliated single-walled carbon nanotubes, a thickener, and optionally a surfactant in a solvent.

[0073] To ensure a uniform mixture of the nanoplatelets and carbon nanotubes, the mixture of step (iii) can be subjected to a high shear mixing stage. Additionally, a further step of compressing the ink (e.g., roll-milling) can be performed to degas the ink, which facilitates printing of the ink onto a substrate.

[0074] The compositions described above can be used as inks for printing on a variety of substrates, including soft polymers (such as polyethylene terephthalate, polypropylene, and polyimides), elastomers (such as silicones and polyurethanes), metal foils and films (such as aluminum, copper, gold, and platinum foils / films), and rigid substrates (such as silicon wafers, glass, quartz, polycarbonate, etc.).

[0075] In addition to the substrates listed above, the inventors have surprisingly discovered that the inks described herein can also be printed onto cellulosic substrate materials such as paper.

[0076] Thus, in a further aspect of the present invention, there is provided a substrate (e.g., a cellulosic substrate) printed with a conductive ink comprising: (i) Carbon nanomaterials (ii) Cellulose derivatives

[0077] The present invention also provides a method of printing a conductive ink onto a substrate (e.g., a cellulosic substrate), the ink comprising: (i) Carbon nanomaterials (ii) Cellulose derivatives

[0078] The conductive ink may include carbon nanotubes and graphite particles or graphite nanoplatelets, among other ingredients, as described above.

[0079] The cellulosic substrate is typically paper or card.

[0080] The ink can be printed using a variety of printing techniques, such as screen printing or inkjet printing.

[0081] The ideal behavior of a screen-printable ink requires a thixotropic rheology profile that undergoes shear thinning within the printing process, followed by elastic recovery and stabilization of the printed structure at the required resolution upon drying or curing. Such behavior is beneficial for high-resolution printing of interconnects and connecting lines for printed electronics. When building electronic circuits suitable for "bare die" or unencapsulated silicon components, print fidelity better than 125 microns is typically preferred for automated die attach methods.

[0082] As mentioned above, the carbon nanomaterial may be graphite nanoplatelets, single-walled carbon nanotubes, or a mixture thereof, and the cellulosic binder may be carboxymethyl cellulose. The film may also include graphite particles as the conductive carbon particles.

[0083] The conductive ink may also have additional ingredients or properties as described herein.

[0084] The present invention combines the high electrical conductivity of nanocarbon material combinations with the thixotropic rheology necessary for good printing properties. Several examples of devices and circuits are presented that demonstrate suitability for printed electronics applications. This example (Example 3) outlines a UHF RFID tag. Similarly, (Example 5) illustrates a microheater device.

[0085] Conductive inks can be used to print a wide range of applications, including but not limited to microwave antennas, RFID tags, biosensing electrodes, printed heaters, wireless induction coils, metasurfaces for tunable low emissivity and reflectivity coatings, strain sensors, surface acoustic wave devices, temperature sensors, energy storage electrodes and electrolytes for supercapacitors, batteries, capacitance sensors, flexible, stretchable or structural electronic conductors, low-density aerogels for catalysis, power storage and chemical repair, self-healing coatings and drug delivery platforms.

[0086] In a further aspect, the present invention provides an RFID tag comprising an antenna deposited (e.g., printed) onto a substrate from the liquid composition described herein. The substrate may be a plastic polymer substrate (such as PET) or a cellulosic substrate (such as paper).

[0087] In a further aspect, the present invention provides a printed heater comprising a heating element printed onto a substrate from a liquid composition described herein.

[0088] Surface strain measurements have many industrial applications. Nanocarbon-based printed structures exhibit strain-dependent conductivity when applied to substrates above the percolation threshold. Polymer binder-based films exhibit reproducible elastic properties, surpassing the use of conductive metals, which may break before reaching the substrate's elastic limit. Therefore, using this invention, measurements of high strains (>2%) on elastic substrates are possible with good reproducibility. Furthermore, this elastic behavior can be extended to modify the antenna's resonant characteristics (frequency and Q factor). A new example is presented in which the resonant behavior of a UHF RF antenna printed on an elastic substrate can be monitored without the need for an internal power source or processing circuitry.

[0089] In a further aspect, the present invention provides a deposition liquid composition as described herein deposited (e.g., printed) onto a stretchable substrate. [Example]

[0090] Example 1 - Exfoliation of Graphite to Form Graphite Nanoplatelets The flake graphite was exfoliated using the apparatus and process described in International Patent Application No. PCT / EP2019 / 077579 to obtain graphite nanoplatelets with an average lateral size distribution of approximately 1 μm and an average thickness of approximately 10 layers.

[0091] Briefly, fine graphite powder (1-50 μm flake size produced by air classification of milled powder) was dispersed in a surfactant-water system and added to the inlet reservoir of a high-pressure homogenizer (such as the apparatus described in International Patent Application No. PCT / EP2019 / 077579). The fluid was then pressurized and accelerated under reduced pressure before exiting the homogenizer's process cell and entering a heat exchanger. Once the fluid was cooled to a temperature maintained by an external cooling system, it was recovered or recycled, depending on the system configuration.

[0092] Once the graphite was processed, the exfoliated mixture was centrifuged at 5,000 g for 20 minutes to remove any unexfoliated crystallites and large debris. All of these parameters were met to deposit nanosheets with only a few layers (i.e., graphite nanoplatelets) remaining. The resulting graphite nanoplatelets had lateral size and thickness distributions ranging from 50 to 2,000 nm and up to 20 nm, respectively.

[0093] Example 2 - Ink Formulation The compositions are shown in the table below for the batches of ink prepared: The total solids of the inks prepared (including binders etc.) was about 3.7% by weight.

[0094] [Table 1]

[0095] To make the ink, the ingredients were weighed into a suitable container. The mixture was heated (hot plate at 60°C) while mixing using a Silverson L5M-A laboratory high shear mixer operating at 5000 rpm to sufficiently reduce the viscosity and mix the ingredients. The mixture was then mixed for 5 minutes.

[0096] The graphite nanoplatelets have a lateral size distribution between 50 nm and 800 nm and a maximum thickness of approximately 20 nm.

[0097] Structural characterization by SEM revealed the presence of a dense network of carbon nanotubes in the gaps between the packed graphite nanoplatelets (see Figures 1 and 2).

[0098] The viscosity of the ink was measured over a shear rate range of 0.1 / s to 100 / s. The ink was found to be thixotropic, and the rheological trace is shown in Figure 15.

[0099] The ink was successfully printed on a variety of substrates, including several grades of polyethylene terephthalate (PET) substrates (DuPont Tejin ST504 & Felix Scholler F40100) and paper substrates.

[0100] The conductivity of the printed films was measured using a four-point probe according to the International Electrotechnical Commission standard IEC TS62607-2-1:2012. The film thickness was measured by SEM cross-section analysis or scanning probe profilometry, and the conductivity and thickness were used to calculate the specific conductivity.

[0101] For printed films, up to 500 kS / m -1 The conductivity was observed.

[0102] Accordingly, the present invention provides highly conductive inks formed from carbon nanomaterials, and in particular highly conductive inks formed from carbon nanomaterials that can be printed on paper substrates.

[0103] (Example 3: Integration of UHF RF tags) A properly designed UHF antenna was screen printed from the ink described in Example 2 above onto a paper-based substrate (standard uncoated label stock) using a 32T mesh screen printer at a dry thickness of approximately 3 microns.

[0104] A photograph of the printed antenna is shown in Figure 3. Figure 4 shows the flexibility of the printed antenna. It can be seen that the printed antenna can be wrapped around a small diameter without compromising the integrity of the printed antenna. Figure 5 shows the printing resolution of the ink.

[0105] The resulting antenna was integrated with a bare-die RFID integrated circuit (Impinj Monza 6 or NXP UCODE 8) via an anisotropic conductive film (ACF) thermode process. The resulting RFID tag was analyzed using a handheld Zebra (model MC3300) reader in an unshielded office environment. The typical read distance achieved was 3 meters.

[0106] Example 4: Printing on a stretchable substrate The adhesive gamma-valerolactone (GVL) was added during the compounding process at 5% by weight relative to the water content of the ink formulation described in Example 2.

[0107] When printed, films printed from this GVL-containing ink exhibited comparable sheet resistance on both PET and paper for the same number of printing passes as the standard formulation (e.g., 2-3 Ω / sq after 3 printing passes).

[0108] The GVL-containing ink was also printed onto two different thermoplastic polyurethane (TPU) elastomers and vulcanized polyisoprene rubber.

[0109] Linear tracks of ink deposited on TPU were used to quantify the ink's response to strain. To perform strain measurements, linear tracks were printed on a dog-bone-shaped substrate attached to a TA Systems Texture Analyzer. Sample resistance was monitored in situ using a Keithley 2614B Source Meter.

[0110] As shown in Figure 6, the film resistor response is linear up to about 5% strain, after which the resistance begins to rise rapidly. The slope of the linear region (shown by the dashed line) is 1, which is interesting given that most isotropic materials exhibit gauge factors (G) greater than 2 upon material deformation. The mechanism by which this occurs has not yet been identified.

[0111] A bowtie-shaped antenna pattern (shown in Figure 7) was printed on a commercially available TPU elastomer substrate using the inks described above for GVL-containing inks. The antenna design was optimized to resonate within the UHF RFID band (860-960 MHz).

[0112] The printed antenna shown in Figure 7 had a read distance of approximately 80-85 cm when fitted with an appropriate RFID integrated circuit (IC).

[0113] To compare the antenna behavior with the resistive response measured in Figure 7, a single antenna was connected to a vector network analyzer (VNA, Pico Technologies PicoVNA 106) via the SMU-A connector and the spectral response was monitored as the antenna was strained. The results are shown in Figure 8, along with interpolated measurements of the resonance position as a function of strain (Figure 9).

[0114] The data show that the antenna resonant frequency response to applied strain is weak, even beyond the region where the film's conductivity is expected to be linear. The data can be equally "fitted" to a constant mean value of 890 MHz. By extrapolating the data on the left side of Figure 6 to 0 Hz, the antenna resistance can be estimated as follows:

[0115]

number

[0116] where S is the extrapolated value of return loss at 0 Hz. The data is plotted in Figure 10 along with that of Figure 6. As can be seen, there is reasonable agreement between the two methods in terms of the relative change in antenna resistance with strain.

[0117] Finally, a single assembled tag (antenna and IC) was tested under repeated strain at 5% and the read range was measured before and after the test. The initial read range was 70 cm, and the final read range (after 10,000 strain cycles) was also 70 cm.

[0118] Example 5: Printable Heater The ink of Example 2 was printed onto label stock paper in the pattern shown in Figure 11. The printing involved three printing passes, and the sheet resistance of the printed film was 2 Ω / □ (ohms per square). The resulting film was flexible, adhered well to the paper substrate, and conformed to a 2 mm roller diameter without peeling.

[0119] When a DC potential difference of 10 V (approximately 0.08 A, 0.8 W) was applied to the printed film, there was a temperature increase of approximately 20°C as measured by infrared thermal imaging (see Figure 12 (A) and (B)).

[0120] Example 6: Graphite-containing ink formulation It was also found that an increase in conductivity was observed with the addition of carbon nanotubes even when micron-sized graphite was used instead of the graphite nanoplatelets of Example 2 above.

[0121] FIG. 13 shows the electrical conductivity of graphite and carbon nanotube mixtures at various mass fractions of carbon nanotubes.

Claims

1. (i) graphite nanoplatelets in the weight range of 0.5% to 3% (w / w); (ii) carbon nanotubes in the weight range of 0.1% to 1.5% (w / w); (iii) a hydrogel-forming thickener selected from cellulose derivatives, wherein the hydrogel-forming thickener is present in an amount of 0.5% to 2% by weight of the total composition; and (iv) water; Including, The liquid composition, wherein the graphite nanoplatelets and carbon nanotubes are present in a weight ratio of 0.15:1 to 0.6:1 (carbon nanotubes:graphite nanoplatelets).

2. 10. The liquid composition of claim 1, wherein the graphite nanoplatelets have a layer count of 20 or less.

3. 3. The liquid composition of claim 1, wherein the graphite nanoplatelets have a thickness of 30 nm or less.

4. 4. The liquid composition of claim 1, wherein the graphite nanoplatelets have an average length of 100 nm or greater.

5. 5. The liquid composition of claim 1, wherein the hydrogel-forming thickener is carboxymethylcellulose.

6. 7. A liquid composition according to any preceding claim, wherein the hydrogel-forming thickener is present in an amount of from 1% to 1.75% (w / w).

7. 7. The liquid composition according to claim 1, comprising single-walled carbon nanotubes having an average diameter of 1 nm to 5 nm.

8. 8. The liquid composition according to claim 1, wherein the carbon nanotubes have a length greater than 3 μm.

9. The liquid composition of claim 1 , further comprising a surfactant.

10. 10. The liquid composition of claim 9, wherein the surfactant is t-octylphenoxypolyethoxyethanol.

11. (a) graphite nanoplatelets in the weight range of 0.5% to 3% (w / w); (b) carbon nanotubes in the weight range of 0.1% to 1.5% (w / w); (c) carboxymethylcellulose in the weight range of 0.5% to 2% (w / w); (d) t-octylphenoxypolyethoxyethanol in the weight range of 0.01% to 0.1%, and (e) water; The liquid composition of claim 1 comprising:

12. 12. The liquid composition of any one of claims 1 to 11, further comprising gamma valerolactone in a weight amount up to 5% (w / w).

13. A cellulosic substrate printed with the liquid composition of any one of claims 1 to 12.

14. 13. A method of applying a conductive film of carbon nanomaterial to a cellulosic substrate, the method comprising printing a liquid composition according to any one of claims 1 to 12 onto the cellulosic substrate.

15. An RFID tag comprising an antenna printed onto a substrate from the liquid composition of any one of claims 1 to 12.

16. A printed heater comprising a heating element printed onto a substrate from the liquid composition according to any one of claims 1 to 12.

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